Scientifically/medically/psychologically, how can you ask or workout/findout how many previous sexual partners a woman has had?

So far I'm asking them if they've had serious relationships before, then they usually say how many years each one was, and I can see their age on the app. If they have a big gap I assume the number is very high. I'm trying different models to estimate the number of partners within those gaps using survey data gathered by governments and various stat orgs on sexual behaviour, education level, demographics etc

I have to make some guesses/assumptions based on profile information and that means the person falls into one distribution over another.

I also wonder if there is a more qualitative way of calculating something like this, like a series of questions that will trick someone into revealing the actual answer without them being aware of what they're really responding to.

*Nothing too expensive like hiring an actor to befriend them and then share stories in some elaborate ruse.